In a magnetic recording/reproducing apparatus such as a hard disk drive (HDD), in order to increase areal recording density, it is important to increase recording capacity. Thus, assisted recording by which the coercive force of the magnetic recording medium is temporarily decreased during recording by applying heat or a high-frequency magnetic field is gaining attention. A system based on the application of high-frequency magnetic field is referred to as “microwave-assisted magnetic recording (MAMR)”.
In MAMR, a strong high-frequency magnetic field of a microwave band is applied to a nanometer-order area to locally excite the recording medium so as to lower the magnetization reversal field when information is recorded. However, because MAMR uses magnetic resonance, a large effect of lowering the magnetization reversal field cannot be obtained unless the high-frequency magnetic field of a high frequency proportional to the anisotropy magnetic field of the recording medium is used. As a countermeasure, Patent Document 1 discloses a high-frequency oscillator for generating a high-frequency assist magnetic field, with a structure similar to that of a giant magnetoresistance (GMR) effect device in which a stacked film is sandwiched between electrodes. The high-frequency oscillator is configured to generate a high-frequency vibrating magnetic field in a localized area by injecting conduction electrons with spin fluctuation generated in the GMR structure into a magnetic material via a non-magnet.
Non-patent Document 1 discloses a technology for recording information in a magnetic recording medium having a large magnetic anisotropy by transmitting a spin-transfer torque from a spin injection pinned layer of an oscillator disposed adjacent to a main pole of a perpendicular magnetic head to an adjacent high-frequency magnetic field generation layer (FGL) via an intermediate layer of Cu, and causing a microwave (high-frequency magnetic field) to be generated by rotating the magnetization of the FGL in a plane at high speed. Further, Non-patent Document 2 discloses a technology for efficiently assisting the magnetization reversal of the magnetic recording medium by disposing the oscillator between the main pole of the magnetic recording head and a trailing shield, and varying the rotating direction of the high-frequency magnetic field in accordance with the polarity of the recording magnetic field.